This invention relates to needle thread feed control apparatus for sewing machines for forcibly performing a needle thread feed operation independently of stitch forming operation. More particularly, this invention relates to a needle thread regulating mechanism for rendering thread cutting or trimming and obtaining a suitable stitch start end.
Various types of prior art sewing machines have been provided with thread cutter or thread trimmer mechanisms which include a thread catch knife mounted downwardly of the needle plate to seize and draw the sewing threads, such as needle and bobbin threads, and a thread trimmer knife, for cutting the threads therebetween.
Such thread cutter mechanisms have been required to properly set an amount of the needle thread to be drawn by the thread trimmer knife, that is, a stroke at which the thread catch knife is moved for starting a next sewing operation in a proper manner. For instance, a lesser amount of needle thread drawn by the thread catch knife results in an insufficient amount of needle thread which is led from the cutting end to an eyelet in the needle through which the needle thread (hereinafter referred to as "residual thread") passes. This will move the cutting end of the needle thread out of the thread at one stitch immediately in the next sewing operation failing to perform sewing operation. Further, a longer residual thread than is necessary either projects from the underside of the work fabric or is entrained in a "bird's nestle" so that the quality of the work fabric is deteriorated.
A thread cutter or trimmer mechanism of this type, which is designed to precisely set the stroke of the thread catch knife has presented difficulty in obtaining a constant length of the thread when it is used under such a condition that the type and thickness of the fabric, the type and denier of the thread, and the speed at which the thread cutter mechanism is moved are changed or varied.
A feed control apparatus as disclosed in Japanese Patent Publication No. Sho 60-48196 and shown in the accompanying drawings, FIGS. 10, 11, and 12, has been proposed to avoid such difficulty to obtain the thread as left with stability, that is, constant in its length.
As illustrated, a lockstitch sewing machine 1 includes an arm 2 which is provided with a needle bar 3 for holding a needle, a thread takeup lever, and a main thread tension unit 5 for applying a predetermined tension to a needle thread as are well known in the art.
The feed control apparatus in the above-mentioned lockstitch sewing machine comprises mainly a feed passage control mechanism A for regulating the passage through which the needle thread passes and for feeding the same, a needle thread pull back mechanism B, and a releasing mechanism C for releasing the needle thread clamped by the main thread tension unit 5 from its pressure. These mechanisms are adapted to cooperate with each other.
More specifically, the feed passage control mechanism A has a solenoid 6 mounted on the front wall of arm 2, a delivery lever 7 rotatable about a screw 8 formed on arm 2 and against the bias of a spring 9, and a first auxiliary thread tension unit 10 mounted on one end of delivery lever 7. The first auxiliary thread tension unit 10 is adapted to clamp the needle thread fed from a needle supply source (not shown) by thread tension discs 10a, 10b back from the main thread tension unit 5. The needle thread pull back mechanism B comprises a second solenoid 11 positioned on the front wall of the arm 2, a lock lever 12 connected to a plunger 11a of the solenoid 11. The lock lever 12 is adapted to rotate about a screw 13 against the bias of a spring 14 when plunger 11a of the solenoid 11 is advanced.
The releasing mechanism C comprises a first release lever 15 which is rotatably mounted by a screw 16 to the front wall of arm 2 and counterclockwise energized by a spring 19, a connecting link 17 which cooperates with delivery lever 7 to rotate release lever 15, and a connecting lever 18.
Numerals 21, 22, and 23 designate a second auxiliary thread tension unit, a sub-thread tension unit, and a second release lever for releasing the second auxiliary thread tension unit 21. Release lever 23 is adapted for movement in association with the connecting link 17. Numeral 14 denotes a check spring mounted to the thread tension unit 5.
Each of the mechanisms and elements or parts of the feed control apparatus is located in an initial position as shown in FIG. 10 during sewing operation, where the thread tension unit 5 is allowed to clamp a needle thread I1 under a predetermined pressure by a pair of the thread tension discs 5a, 5b, and a release plate 10c is interposed between the thread tension discs 10a, 10b of the first auxiliary thread tension unit 10 while a release lever 23 is inserted between thread tension discs 21a, 21b of the second auxiliary thread tension unit 21. With this arrangement, both of the auxiliary thread tension units 10, 21 assume a state in which the needle thread is released whereas the auxiliary thread tension unit 22 is in a condition to normally hold the thread needle.
A thread cutting signal is output after the sewing operation to operate the solenoid 6 just before the thread cutting is initiated by the thread cutting mechanism (not shown), rotating counterclockwise the delivery lever 7 to hold the apparatus in the condition as shown in FIG. 11. In this connection, it may be noted that the first auxiliary thread tension unit 10, movable with the delivery lever 7, is kept away from the release plate 10c to clamp the needle thread I1 to then draw the same out of the side of the thread supply source. As a result, the needle thread I1 located in the range from the first auxiliary tension unit 10 to the needle is slackened to an extent corresponding to the amount of thread as drawn. At this moment the needle thread as drawn by the first auxiliary tension unit 10 amounts to L+.alpha., wherein L is the length of needle thread required for next sewing operation, and .alpha. is some play.
Under the condition in which the needle thread is let out, the threat cutting operation is performed. For this reason, the thread cutting mechanism is adapted not only to smoothly feed the needle thread with the thread catch knife but also to readily cut the needle thread by the thread cutter knife. In this connection, it is noted that the delivery lever 7 urges back the lock lever 12 against the bias of spring 14 due to its counterclockwise rotation and then engages tongue 12a of the lock lever 12 so that the lever 7 may be maintained in a position where it is rotated even if the solenoid is deactuated.
Now, the next sewing operation is initiated, solenoid 11 is actuated between the first stitch and a couple of stitches to clockwise rotate the lock lever 12, thus releasing one end of the delivery lever 7. This will clockwise rotate the delivery lever 7 to a position where it is located (FIG. 12) between the initial position as shown in FIG. 10 and the delivery position as shown in FIG. 11 by engagement of the stopper 12a on lock lever 12 with the end of the connecting lever 18.
In this manner, the delivery lever 7 is moved to a position between the initial position and the delivery position so that the needle thread is stretched in a position where the needle is positioned is pulled back by the first auxiliary thread tension unit 10 to the needle thread supply source. The needle thread length to be pulled back would be the length about one-half of the play. The half of the thread, as residual of the play .alpha., is pulled up by up-movement of the thread takeup lever. This will leave a thread amount one-half of the length L at the beginning of sewing operation performed in the work fabric. The conventional apparatus has been effective in preventing the needle thread I1 from falling out of the needle at the beginning of the sewing operation and avoiding not only formation of the bird's nestle at the beginning of the sewing operation on the work fabric but also excess length of residual thread.
Notwithstanding, the conventional apparatus is arranged so that the needle end thread left by thread cutting operation is pulled back during several stitches after the next sewing operation to make an accurate pull-back operation impossible. More specifically, the pull-back operation, if performed after the sewing operation, is immediately followed by the needle thread draw-out operation performed by the needle, hook and the like one after the other. Subsequently, the needle thread is subjected to considerable elongation and frictional resistance to readily vary the amount of thread pulled back. The needle thread is required to perform the pull-back operation synchronously with driving the sewing machine complicating regulation and control. The overall works are rather intricate because the prior art apparatus is such that the needle thread is fed and pulled back by moving the first auxiliary tension unit 10. This necessitates adjustment of the clamping force by taking into account the type of the needle thread I1 to be applied and the force exerted by the main thread tension unit 5 to the thread. More specifically, for positive needle thread delivery and pull-back operation, a clamping force as high as possible is set so as not to allow alignment of the needle thread out of a position where it is clamped by the first auxiliary thread tension unit 10. However, if the needle thread clamping force is greater than the force exerted on the needle thread by the main thread tension unit, tension on the upper thread is considerably high at the time when sewing operation is initiated failing to obtain proper tightness of stitches and stitch perforation.
In view of the foregoing, the force applied to the upper thread I1 by the first auxiliary thread tension unit 10 should be set in such a manner that it is lower than the force applied to the upper thread I1 by the main thread tension unit 5 on one hand, and the needle thread I1 is not out of alignment within the first auxiliary thread tension unit on the other hand. This will require that the apparatus involve much difficulty and great complexity in adjustment and setting of the value of the force. If a change in the type of the needle thread is required, a low productivity is involved in the conventional apparatus if it is in use.